This invention relates to an illumination system and, more particularly, to an extra high output (EHO) amalgam fluorescent lamp and an associated control system to operate the lamp under optimum operating conditions.
Low pressure, mercury vapor fluorescent lamps are used in a variety of lighting applications. Of particular interest, for purposes of the present invention, is the widespread use of fluorescent lamps to illuminate documents being copied in a reprographic device.
In a conventional mercury fluorescent lamp, an electrical discharge is generated in a mixture of mercury vapor at low pressure and a fill gas typically argon, neon, Krypton, xenon or mixtures thereof. The light output from the lamp depends, among other variables, on the mercury vapor pressure inside the lamp tube. It is known in the prior art that the optimum mercury pressure for maximum light output of a fluorescent lamp approximately 7 mtorr (independent of current) which corresponds to a mercury cold spot temperature of 35.degree. C. At this temperature and pressure, the light output increases monotonically with the current. At cold spot temperatures higher or lower than the optimum, light output falls off. It is therefore desirable to maintain the mercury pressure at the optimum at any lamp current and at any ambient temperature. Prior art techniques for accomplishing this function typically require a temperature-sensitive device such as a thermocouple, thermistor or thermostat to monitor the temperature of the cold spot. A feedback circuit provided closed loop control of a temperature-regulating device to maintain the optimum mercury pressure.
For certain document reproduction applications, it is desirable to operate the illumination source at extremely high loadings. In the prior art applications mentioned above, the power loading is typically 40 watts, whereas power loadings up to 120 watts may be required for certain applications. At this increased loading, the lamp wall temperature is greatly increased, requiring the use of active cooling devices such as fans and the like. Additionally, the lamp is very sensitive to its axial thermal temperature profile. Deviation from optimum can cause wide variation in light output along the length of the lamp.
In order to achieve better thermal control of a fluorescent lamp at high loading, it is known to incorporate an amalgam-forming material such as an indium patch, within the lamp envelope. The indium forms an amalgam with the mercury, thus chemically containing the mercury within the amalgam. The temperature at which mercury is released from the amalgam is significantly higher (approximately 100.degree. C.) than the optimum lamp wall temperature of the conventional non-amalgam lamp (35.degree. C.). (The actual temperature is adjustable by the amalgam material composition.) Thus, use of the amalgam fluorescent lamp eliminates the need for active cooling devices. However, there is a need for a control system to control the optimum thermal operating point of the lamp. The present invention is directed towards a control system which controls the temperature at a profiled power density lamp heater sleeve and adjusts the input power to the lamp through a feedback circuit.
More particularly, the invention relates to a monitoring and control system for an amalgam fluorescent lamp, said system including a multi-element lamp heater sleeve adapted to control the amalgam temperature and to provide a non-uniform power density axially across the lamp and control means for sensing temperature along a plurality of areas of said heater sleeve and for adjusting the temperature at each of said areas.
The following prior art publications have been identified as disclosing various types of temperature control means for non-amalgam type of fluorescent lamps.
U.S. Pat. No. 3,779,640 to Kidd discloses temperature control means for a fluorescent lamp used in electrophotographic printing. Control means include a heater sleeve and blower responsive to a thermostat positioned on the lamp wall. Circuit means in conjunction with the thermostat can de-energize the heater sleeve to maintain the lamp temperature within a 10.degree. F. range.
Japanese Pat. No. 61-217033 to Tanaka discloses a temperature-sensing element on the wall of a fluorescent photocopier lamp. When the sensing element detects the lamp to be at a preset temperature, the copier turns on automatically.
Japanese Pat. No. 59-42534 to Ishikawa discloses a fluorescent lamp with separate heaters at its center and ends. A patch in the center of the lamp detects temperature variation and separate controls allow the heat supply to the ends to be adjusted when their temperature rises.